1. Field of the Invention
The present invention relates to refrigeration devices for cooling and storing material at low temperatures. More particularly, the invention relates to a low temperature refrigeration device incorporating vacuum insulation panels and a pressure equalization sub-system for ensuring reliable cold storage of materials such as perishable biological samples.
2. Description of the Prior Art
Low temperature refrigeration devices, especially ultra-low temperature freezers, are commonly used in laboratory settings for cooling and storing materials, including biological samples, between approximately xe2x88x9240xc2x0 C. and xe2x88x9295xc2x0 C. The ambient temperature in areas in which these devices are typically used can reach 32xc2x0 C. Because of the great difference between interior and ambient temperatures, and the typically perishable and sometimes irreplaceable nature of the contents, mitigating heat transfer in order to maintain the required interior temperature is of paramount importance. Proper insulation is essential in mitigating heat transfer.
Existing insulation schemes for low temperature refrigeration devices combine thin, high-cost vacuum insulation panels (VIPs) with thick, low-cost conventional blown foam insulation to achieve a desirable balance of cost and size. Many such schemes, however, rely on a few large-sized VIPs, typically one for each side of the refrigeration device""s rectangular housing. Unfortunately, this means that failure of a single VIP results in a very large area of decreased insulation and potential heat transfer. Furthermore, insulation schemes that use multiple smaller VIPs, rather than a single large VIP, typically experience increased heat transfer at imperfect seams where adjacent panels butt against one another.
Another problem with prior art insulation schemes relates to interior and exterior pressure differentials. Opening and closing the refrigeration device""s door results in warm air being introduced into and trapped within the interior compartment. As the warm air cools, a lower than ambient pressure develops in the interior compartment. This pressure difference can have varied effects, including stressing the relatively weak VIPs, possibly resulting in a loss of vacuum, and making the door more difficult to subsequently open.
The refrigeration system of the present invention presents novel improvements in the art of refrigeration technology, thereby enabling more reliable and efficient cooling and low-temperature storage of materials such as biological samples. The refrigeration system strategically combines conventional, high-density, foamed-in-place urethane insulation with VIPs to economically decrease wall thickness from five inches to two inches, which allows for greater interior storage space without increasing exterior cabinet dimensions. Thus, a refrigeration unit built according to the preferred embodiment has approximately twenty percent more storage capacity than the same size 20.2 cubic-foot conventional upright device. Using numerous smaller VIPs rather than a few large VIPs mitigates the effects of panel failure, thereby localizing and minimizing undesired heat transfer. Furthermore, each insulative panel has beveled edges to overlap and positively interface with adjacent panels, which more reliably impedes heat transfer along the interface seam.
A pressure equalization port is included to equalize differences between interior and ambient pressures, which reduces this source of stress on the VIPs and thereby extends their useful life and ensures more reliable cooling and storage. Furthermore, equalizing pressures makes opening the refrigeration device""s door easier. A heating element keeps the pressure equalization port frost-free, thereby helping to maintain its operation and efficiency.
These and other important aspects of the present invention are more fully described in the section below entitled DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT.